Since Spear et al. succeeded in doping an amorphous silicon (that is, a-Si:H), a number of photovoltaic elements with the use of a-Si:H were proposed, for examples, as disclosed in U.S. Pat. No. 4,064,521.
Nowadays, a number of a-Si:H solar cells have been employed in watches, miniature computers, street lamps, etc. as their power sources.
However, any of those a-Si:H solar cells is still problematic especially in photoelectric conversion efficiency, anti-photodegradation property, and the like.
In order to provide a a-Si:H solar cell having an improved photoelectric conversion efficiency and an improved anti-photodegradation property, a variety of proposals have been made.
For example, there have been reported a solar cell provided with a p-type semiconductor layer composed of an amorphous silicon carbide having a wide forbidden band as the window layer through which light is impinged (Y. Tawada et al., Japan J. Appl. Phys. 20(1981) Supplement 20-2, 219): a solar cell provided with a less light absorptive n-type semiconductor layer composed of a microcrystal silicon material as the window layer through which light is impinged [Y. Uchida, US-Japan Joint Seminar, Technological Applications of Tetrahedral Amorphous Solids, Palo Alto, Calif. (1982)]; and a solar cell provided with a p-type semiconductor layer composed of a microcrystal silicon carbide as the window layer through which light is impinged (Y. Hattori et al., Technical Digest of the International PUSEC-3, Tokyo, Japan, 1987, A-II, a-3).
Further, a proposal has been made to incorporate a trace amount (less than 10 ppm) of phosphorus atoms (P) or boron atoms (B) into the i-type semiconductor layer in order to increase carrier range in said i-type semiconductor layer (W. Kusian et al., The conference record of the nineteenth IEEE photovoltaic specialists conference-1987, p. 599; and M. Kondo et al., The conference record of the nineteenth IEEE photovoltaic specialists conference-1987, p. 604).
However, in the case of such solar cell having the window semiconductor layer composed of the foregoing microcrystal silicon material, microcrystal silicon carbide or amorphous silicon carbide, there are problems that interface-state and discontinuities in the conduction band and/or the valence band such as notch, spike, etc. which lead to shortening the lifetime of photo-excited carrier often occur at the interface between the i-type semiconductor layer and the window semiconductor layer especially when there is a difference between the window semiconductor layer and the i-type semiconductor layer with respect to their layer structure, forbidden band and amount of a dopant contained, to thereby provide negative influences for carrier generation in the vicinity of the interface between the i-type semiconductor layer and the window semiconductor layer and for transportation of the carrier generated.
It is considered that the problem relating to the lifetime of the photo-excited carrier can be somewhat eliminated by the incorporation of a trace amount of phosphorus atoms (P) or boron atoms (B) into the i-type semiconductor layer as found in the above report. However, other problems relating to the discontinuities in the conduction band and valence band such as notch, spike, etc. still remain unsolved.
In order to eliminate the problem relating to the foregoing discontinuities, there is a proposal to continuously vary the composition of the i-type semiconductor layer toward the window semiconductor layer as disclosed in Japanese Unexamined Patent Publication 55(1980)-11329. However, said problem cannot be satisfactorily eliminated by this proposal mainly because of a difference between the condition for the formation of the i-type semiconductor layer and that for the formation of the window semiconductor layer.